There are various problems as described hereunder in conventional FRP curved pipes such as FRP rackets and frames of bicycles.
For example, in a conventional process for producing a racket, after cutting a woven fabric prepreg or a unidirectional prepreg of reinforcing fibers such as carbon fibers or glass fibers or a cross-stacking prepreg formed by crossing and stacking the unidirectional prepregs at a predetermined width, the cut prepregs are wound and stacked one layer by one layer around a core material coated with an expansive tube made from resin or rubber (a tube made from a film-like material which has an expansion property), as described in JP-A-HEI-5-96030. Then, the core material is removed, a preform obtained is placed in a mold, and while the mold is heated, the preform is pressure-molded by injecting a pressurized medium (for example, a pressurized air) into the inside of the expansive tube (generally, called "internal pressure molding").
In such an internal pressure molding, the circumferential length of the periphery of the preform is set to be less than the circumferential length of the inner surface of the cavity of the mold so as to facilitate charge of the preform into the cavity. When heated and formed, the preform is pressed onto the inner wall surface of t he cavity by the pressure of the pressurized medium introduced into the expansive tube.
In such a conventional process, however, as shown in FIGS. 42 and 43, because prepregs 203 are wound and stacked one layer by one layer around a core material 202 coated with an expansive tube 201, a stacked portion (overlapped portion) is generated on each layer. Although all the layers must move (shift) in the circumferential direction ultimately at the time of heating and pressure molding in the mold, the movement is not completed in some portions because of the presence of many stacked portions. There fore, there occurs a case where an outer layer cannot reach the wall surface of the cavity, or a desired pressing force onto the wall surface cannot be generated even if the outer layer can reach the wall surface. Molding defects such as voids and lack of resin are frequently generated on an outer surface of a molded product from such causes. Therefore, there is a problem that nonuniformity of the surface quality and the strength of the molded product increases.
Moreover, in the formation of the preform, because prepregs 203 are wound one layer by one layer around the core material 202, the working efficiency is very bad.
In the above-described process, a molding in the mold is performed, for example, as shown in FIG. 44. In the figure, numeral 204 indicates an expansive tube after core material 202 is removed, numeral 205 indicates a preform, numerals 206 and 207 indicate a mold, and numeral 208 indicates a parting line. Numeral 209 indicates a core which forms a groove for gut strings of a racket and retreats at the time of mold opening.
A racket is generally constructed from complicated curved surfaces and molded in a closed cavity. Therefore, voids or pinholes 210 are likely to occur on a portion present at the farthest position from the meeting surface 208 of the mold and having a small curvature, as shown in FIG. 45. These defects must be filled with a putty, and this treatment deteriorates the productivity.
Further, in the conventional process, because an internal pressure is given to a preform composed of only woven fabric prepregs, unidirectional prepregs or a cross-stacking prepreg and having a high density of reinforcing fibers at the surface layer and the preform is heated and molded, when the internal pressure is given, the reinforcing fibers with a high density adhering to the inner wall surface of a cavity deteriorate the flowability of a resin, and this causes the deaeration to be damaged. If the resin flowability is reduced and the deaeration property is damaged, the above-described voids and pinholes 210 are likely to occur and the surface quality deteriorates.
Further, in the conventional internal pressure molding, a relatively low pressure (for example, not more than 0.8 MPa in a case of compressed air) is applied for the reasons described later. Therefore, there occur such problems as that a preform cannot be sufficiently adhered to an inner wall surface of a cavity and that voids are generated or resin lack occurs on some portions because the air in the cavity cannot escape completely.
In the production of a preform for a curved pipe such as a racket, there are a process wherein prepregs are wound by hand around a core material coated with an expansive tube one layer by one layer and in a predetermined order such that the orientation directions of the reinforcing fibers thereof are set to desired directions, and a rolling table process wherein prepregs are arranged on a table and the arranged prepregs are wound around a core material coated with an expansive tube by rolling the core material on the arranged prepregs.
In such processes, however, the working efficiency is bad, the working needs a long time, the quality obtained is not stable and automization thereof is difficult. If such a process is present in a process for producing a racket, the quality of molded rackets is not uniform and it causes increase of production cost.
In the internal pressure molding, generally a relatively low pressure is applied as aforementioned. If the pressure of a pressurized medium pressing a preform from the inside of the preform is low, on a surface, particularly on a surface of a molded product such as a racket constructed from complicated surfaces, a problem is likely to occur that voids and lack of resin originating from poor adherence of the preform to an inner wall surface of a cavity deteriorate the surface quality of the molded product. The part of the pressurized medium is to press the preform onto the inner wall surface of the cavity during the time until a resin is thermoset. As a concrete medium, usually a gas is used as JP-A-SHO 56-166862 exemplifies a nitrogen gas having a pressure of not less than 0.6 MPa and JP-A-SHO-53-9643 exemplifies a compressed air having a pressure of not lower than 3 MPa.
In such a process utilizing a gas as the pressurized medium, however, although the connection between an expansive tube and an external pressurizing apparatus is easy and a bad affect such as occurrence of soil of a molded product is less even if the gas leaks by a bad condition of the connection, a danger in handling is accompanied if the pressure is over 1.5 MPa because a gas is a compressible fluid, and in addition, the cost for the apparatus therefor is very high. Therefore, it is not practical to suppress surface defects by pressurizing the gas to a high pressure. Accordingly, a gas with a relatively low pressure has been employed. However, when the low-pressure gas is used, surface defects based on the low pressure are inevitably generated, and the cost for correcting the surface defects by post processing increases and this causes increase of the production cost.
Accordingly, it is expected that a process for using an incompressible fluid such as an oil is applied instead of such a process for using a compressible fluid such as a gas. However, because such an incompressible fluid gives a very great affect to the productivity when it leaks, a concrete means for applying such a process is not disclosed at all even in J-PA-SHO 56-166862.
Further, in the internal pressure molding, because a good adherence property of a preform to an inner wall surface of a cavity is required, the preform is desired to have a property for easily fitting the inner wall surface (a good fitting property). For such a requirement, a technology of internal pressure molding using a preform composed of a prepreg prepared by impregnating a resin into a tubular multi-layer braid is disclosed in JP-B-HEI 5-80329.
In this process, however, there is a problem that, because a non-solvent system resin is used and such a resin has a high viscosity, the resin is hard to be impregnated into the braid, and particularly, as the number of multiple layers of the braid becomes large, the resin is harder to be impregnated. Further, in order to facilitate its impregnation of its resin, a resin having a low viscosity must be selected, and therefore, the freedom of the selection of the resin is small. If the resin has a high viscosity, the production speed must be reduced. Moreover, there is a problem that the irregularity of the surface of the braid is large as compared with that of a unidirectional prepreg and therefore defects such as voids and pinholes are likely to occur on the surface of a molded product. Furthermore, although the braid is formed in a stretched condition depending upon the tension at the time of braiding, the braid has an expansive property because the structure thereof is loose, and when the tension is released, the elongation of the braid is recovered and the braid becomes thicker, and therefore the thickness and the length cannot be stable.
The above-described JP-B-HEI 5-80329 discloses a method for forming a tubular braid having a triaxial structure in order to solve the problem originating from the expansive property of the braid. However, although such a triaxial-structure braid can solve the problem of the expansive property in the axial direction, because the expansive property in the circumferential direction reduces at the same time, the fitting property to an inner wall surface of a cavity, which is a merit on use of the braid, is greatly injured.
On the other hand, as the process for producing an FRP curved pipe such as a tennis racket, so called resin transfer Molding is known, for example, as described in U.S. Pat. No. 3,755,037. In the process described in U.S. Pat. No. 3,755,037, a pressure-resistance tube is coated on a mandrel made by gathering steel wires, after helical winding layers of reinforcing fibers are formed on the mandrel, the mandrel is pulled off from the layers to form a preform, the preform is placed in a cavity of a lower mold which is formed as a required shape of a racket, an upper mold is closed, and without pressure reduction of the inside of the cavity, a resin is injected into the cavity to mold a racket by impregnating the resin into the preform while the inside of the tube is pressurized.
In this process, however, because the injection of the resin is performed without pressure reduction of the inside of the cavity, air present in the cavity cannot escape completely, and voids and pinholes are likely to be generated on the surface of the obtained racket.
Further, in a process disclosed in JP-A-HEI 3-176083, after a preform of reinforcing fibers is placed in a cavity of a mold, molten .omega.-lactam containing a catalyst for polymerization and an initiator is injected into the cavity, and it is heated and polymerized to mold a racket whose matrix is a polyamide resin.
In this process, however, because the polymerization reaction in the cavity accompanies a rapid reaction, setting of the amount of the initiator and the temperature is difficult, controlling of the degree of the polymerization and the time of the reaction is difficult, and therefore voids are likely to be generated. If voids or pinholes are generated on the frame surface, the quality of the surface is damaged, and if the content of the resin varies depending on the surface portions, the characteristics reduce or the weight balance cannot be maintained, and therefore the value of the product reduces.